Britons are under-reporting how many calories they consume by up to 50%, a new study suggests.

Many national surveys conclude that the average adult consumes around 2,000 calories but new research suggests it could be almost 3,000 calories.

The authors, from the part-privatised government agency the Behavioural Insights Team (BIT), wanted to examine why official statistics show calorie intake has declined over recent decades, despite rising levels of obesity.

They concluded that the fall can be explained by under-reporting of calorie intake.

This could be driven by a range of factors including people not revealing the true extent of their calorie consumption and increased snacking and eating outside the home - which makes calorie intake harder to track, they suggested.

Their new report, Counting Calories, concludes that if the nation was consuming the number of calories that it reports, people on the whole would be losing weight instead of gaining it.

Meanwhile, they added that declining levels of physical activity are not a "realistic explanation" for the increase in obesity.

Strategies to reduce obesity should focus on reducing calorie consumption, the authors said.

Michael Hallsworth, co-author of the paper and director of health at the Behavioural Insights Team, said: "Counting Calories suggests that strategies to reduce obesity should focus on reducing calorie consumption.

"Our analysis shows that it's unlikely that calorie intake has dramatically decreased in recent decades. Instead, it seems we are reporting our consumption less accurately.

"We should look at new ways of helping people report what they eat."

Figures from the Health and Social Care Information Centre show that, in 2014, 58% of women and 65% of men in England were overweight or obese.